1. Typical Zinc Oxide Varsity
Generally, a zinc oxide varistor is provided as a polycrystalline zinc-oxide ceramics. Specifically, the zinc oxide varistor has been produced by mixing zinc oxide powder, transition metal oxide powder and bismuth oxide powder, and burning the mixture at a high temperature, to form a polycrystalline body with a structure in which a bismuth oxide or the like is segregated in the boundaries between zinc oxide grains each containing a transition metal oxide dissolved therein in the form of a solid solution (see, for example, the following Non-Patent Publication 1).
An appropriate additive makes it possible for the zinc oxide ceramics to exhibit a nonlinear current-voltage characteristic in which each grain boundary in the zinc oxide ceramics has an operating voltage of about 3 V (see, for example, the following Non-Patent Publication 2). That is, the operating voltage as one of varistor characteristics is generally determined by the number of grain boundaries. Specifically, as shown in FIG. 7, in a varistor device comprising two electrodes 2A, 2B provided at opposite end surfaces thereof and zinc oxide (ZnO) grains 1 residing therebetween, an overall operating voltage of the varistor device is determined by the product of the number of boundaries between the zinc oxide grains 1 and the operating voltage having the nonlinear current-voltage characteristic in each of the grain boundaries. Thus, the size of the zinc oxide grains 1 in the zinc oxide ceramics inherently contributes to the varistor characteristics.
The grain size of a ceramics depends on an additive and a burning temperature, and generally has a statistical distribution. This causes difficulties in setting the number of grains or each size of grains in a ceramics at a predetermined value. Therefore, the production of a low-voltage type varistor essentially requires a particular technique in addition to a simple burning technique for forming a ceramics. For example, a varistor device having an operating voltage of 30 KV is required to ensure 10000 grain boundaries each having an operating voltage of 3 V. In contrast, a varistor device having an operating voltage of 6 V means a ceramics which includes 2 grain boundaries each having an operating voltage of 3 V, or only 3 grains. That is, some technique for forming a ceramics having a small number of grain boundaries is required to produce a varistor operable at a low voltage.
2. Multilayer Varsity
As one technique for producing a varistor having a small number of grain boundaries, or a low operating voltage, there has been known multilayering (see, for example, the following Patent Publication 1). This technique comprises preparing a sheet-shaped compact to be burned as a ceramics, and forming alternate layers of a metal electrode layer and a zinc-oxide ceramics layer, as shown in FIG. 8(A). The intervening metal layer makes it possible to hinder the contact between the zinc oxide grains or reduce the number of grain boundaries so as to achieve a varistor having a low operating voltage.
However, as seen in the enlarged view of FIG. 8(A), while each of a grain boundary 1 and a grain boundary 2 of the ZnO grains 1 extending in a direction orthogonal to a current path contributes to varistor characteristics, a grain boundary 3 extending parallel to a current path would be unnecessary in view of enhancement of varistor characteristics. Moreover, a current flowing along a current path P1 comes across three grain boundaries, and a current flowing along a current path P2 comes across four grain boundaries. This means that an operating voltage of the varistor differs between the current paths P1, P2. Thus, a varistor operation at lower voltage can be achieved only if the number of grain boundaries is more strictly controlled.
3. Single-Grain-Boundary Varsity
It is known that each grain boundary of a zinc oxide varistor containing an appropriate additive has an operating voltage of 3 V. Thus, a low-voltage type varistor having any operating voltage of an integral multiple of 3 V can be produced by preparing a plurality of varistors each with a single grain boundary, and connecting them in series.
There has been known a single-grain-boundary varistor experimentally produced by forming a bismuth-containing oxide crystal phase intervening between zinc-oxide single crystals (see, for example, the following Non-Patent Publication 3). While this technique can achieve a current-voltage characteristic with high nonlinearity, it still involves a problem about strength of a junction between the opposed zinc-oxide single crystals. The single-grain-boundary varistor produced through a process utilizing a crystalline grain-boundary layer as disclosed in the Non-Patent Publication 3 leaves a problem about mechanical strength of a junction therein, as in the after-mentioned Comparative Examples.
There has also been known a single-grain-boundary varistor device obtained by fundamentally joining zinc-oxide single crystals together without forming an intervening crystal phase between grain boundaries (see, for example, the following Non-Patent Publications 4 and 5). While a certain level of mechanical strength is achieved in the varistor device disclosed in the Non-Patent Publication 4, the varistor device without any intervening grain-boundary layer has a poor performance, wherein an α-value as a performance index of varistor characteristics is less than 10. Similarly, adequate varistor characteristics are not achieved in the varistor device disclosed in the Non-Patent Publication 5, due to no intervening grain-boundary layer. However, the Non-Patent Publications 4 includes a valuable suggestion that, while a nonlinear current-voltage characteristic is achieved by joining zinc-oxide single crystals each containing manganese and cobalt dissolved therein in the form of a solid solution, no nonlinear current-voltage characteristic is achieved if single crystals without addition of manganese and cobalt are joined together.
[Patent Publication 1] Japanese Patent Laid-Open Publication No. 10-270214
[Non-Patent Publication 1] M. Matsuoka, Jpn. J. Appl. Phys. 10, 736-746 (1071)
[Non-Patent Publication 2] “Evaluation of Single Grain Boundaries in ZnO: Rare-Earth Varsity by Micro-Electrodes” S. Tanaka, K. Takahashi; “Key Engineering Materials Series, Vol. 157-158, CSJ Series Vol. 1, (Electroceramics in Japan I)” p 241 (1998), (Trans Tech Publication, Switzerland)
[Non-Patent Publication 3] “MODEL EXPERIMENTS DESCRIBING THE MICRO-CONTACT OF ZnO VARISTORS”, SCHWINGU, HOFFMANN B, JOURNAL OF APPLIED PHYSICS, 57 (12): 5372-5379 (1985)
[Non-Patent Publication 4] “Synthesis of ZnO bicrystals doped with Co or Mn and their electrical properties” Ohashi N, Terada Y, Ohgaki T, Tanaka S, Turumi T, Fukunaga O, Haneda H, Tanaka J, JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPER SHORT NOTES & REVIEW PAPERS, 38 (9A): 5028-5032, SEPTEMBER (1999)
[Non-Patent Publication 5] “Current-voltage characteristic across [0001] twist boundaries in zinc oxide bicrystals” Sato Y, Oba F, Yamamoto T, Ikuhashi Y, Sakuma T, JOURNAL OF THE AMERICAN CERAMIC SOCIETY, 85 (8): 2142-2144, AUGUST (2002)